Purity % vs Net Peptide Content: The Number Under the Headline

A lyophilized peptide vial holds the peptide — and, unavoidably, several other things that are neither impurities in the HPLC sense nor signs of a bad product:

• Counterions. Peptides are charged molecules and cannot exist as bare ions in a solid; their basic sites pair with anions. Material purified by standard methods carries trifluoroacetate from the cleavage and purification chemistry, and TFA salts routinely account for a noticeable fraction of mass — for arginine- and lysine-rich sequences, easily 10–20%.
• Residual water. Lyophilization reduces water to low levels but not zero; a few percent bound moisture is normal.
• Residual salts and solvents. Small amounts from buffers and processing.

None of these appear as peaks in a standard HPLC purity run — and that fact is the whole story.

As covered in the HPLC article, peptide purity is measured by UV absorbance at ~214 nm — the wavelength where the peptide bond absorbs. The detector therefore reports peptide-bond-containing species and very little else. Trifluoroacetate has no peptide bond. Water has no peptide bond. Salts have no peptide bond. They pass the detector invisibly, contribute zero peak area, and are excluded from the purity calculation by construction.

So a purity percentage is a ratio within the detected peptide population:

HPLC purity = main-peak area ÷ total peak area of detected species

It is a statement about the peptide fraction's composition — and a genuinely important one, because it quantifies deletion sequences, oxidation products, and other synthesis-related contaminants. What it is not is a statement about vial mass.

Net peptide content (peptide content, or assay) answers the mass question: what percentage of the gross material is peptide. It requires methods that respond to the amount of peptide rather than its proportion among peaks:

• Amino acid analysis (AAA) — the reference method: the sample is completely hydrolyzed into its constituent amino acids, which are quantified individually against standards. The summed residues give the absolute peptide quantity, and dividing by sample mass gives content. As a bonus, the residue ratios independently check the sequence composition.
• Elemental (nitrogen) analysis — peptides have a calculable nitrogen fraction; combustion analysis of total nitrogen back-calculates peptide mass.
• Quantitative UV/spectrophotometry — usable when the sequence contains strongly absorbing residues (tryptophan, tyrosine) with known extinction coefficients.

Typical net peptide content for research-grade lyophilizates runs broadly in the 60–90% range depending on sequence, counterion load, and moisture — numbers that surprise readers who assumed "99% pure" meant 99% of the mass.

The two multiply, and the product is the operative quantity: actual target peptide = vial mass × content × purity. A 10 mg vial at 80% content and 99% purity contains ~7.9 mg of the target molecule — a calculation any quantitative experimental design has to run, and one that neither number supports alone.

For experimental accuracy, content is the number that sets real concentrations. Two vials of identical purity but different counterion loads deliver different molar amounts from the same weighed mass; work that depends on precise concentrations (binding constants, EC50 determinations of the kind reported throughout the receptor-comparison literature) is calibrated on content, not purity.

For judging material quality, purity is the number that reports whether synthesis and purification did their job — it is where deletion sequences and degradation products show up, and it is rightly the headline figure on a Certificate of Analysis.

For reading vendor claims, the distinction is a fast competence test. A listing quoting purity as if it described mass — or content as if it described impurities — is conflating measurements that professional documentation keeps separate. And a certificate that reports purity and identity while remaining silent on content is not hiding anything; it is simply reporting the tests that were run, which is exactly how the COA article recommends reading any certificate: by what was measured, never by what a strong number seems to imply about measurements that weren't.

HEEZ documentation reports independent third-party HPLC purity and mass-spectrometry identity; Certificates of Analysis are published on product pages as testing is completed — review the report before you order.